Return to BSD News archive
Path: sserve!newshost.anu.edu.au!munnari.oz.au!foxhound.dsto.gov.au!fang.dsto.gov.au!myall.awadi.com.au!myall!blymn From: blymn@awadi.com.au (Brett Lymn) Newsgroups: comp.os.386bsd.development Subject: Re: V86 mode & the BIOS (was Need advice: Which OS to port to?) Date: 15 Aug 1993 06:30:11 GMT Organization: AWA Defence Industries Pty. Ltd. Lines: 30 Message-ID: <BLYMN.93Aug15160011@siren.awadi.com.au> References: <3390@vall.dsv.su.se> <CB11pL.D12@imag.fr> <107181@hydra.gatech.EDU> <1993Aug4.073826.24956@fcom.cc.utah.edu> <107725@hydra.gatech.EDU> <245jrfINNrc0@bonnie.tcd-dresden.de> <108457@hydra.gatech.EDU> <1993Aug12.110032.16307@cnplss5.cnps.philips.nl> NNTP-Posting-Host: siren.awadi.com.au In-reply-to: rooij@bashful.isp.cft.philips.nl's message of Thu, 12 Aug 1993 11:00:32 GMT >>>>> On Thu, 12 Aug 1993 11:00:32 GMT, rooij@bashful.isp.cft.philips.nl (Guido van Rooij) said: Guido> gt8134b@prism.gatech.EDU (Howlin' Bob) writes: >As for slow task switching, the 386 doesn't access the I/O permissions >bitmap until an I/O instruction occurs. The size of the bitmap has >no effect on task switching performance. Guido> The reason this was brought up is the fact that on task switching you Guido> also need somehow to get the io permission bitmap in the TSS. Since Guido> 386bsd does not use the builtin taskswitching of the 386, theis bitmap Guido> has to be memcpy'd into the TSS, resullting in slow task switches. I do not think that we need to copy the i/o bitmap at all. There are a couple of alternatives. One is modify the TSS location on a task switch to point at a different i/o bitmap. Each task need not have it's own i/o bitmap, they can use the default one until they request a change - then the bitmap could be copied and updated as requested (sort of a copy on write scheme). The other alternative is to actually use a TSS for each process and do a jump through the TSS. According to Bill Jolitz the only reason for the scheme we have at the moment is it only saves 8 registers instead of 20, the scheme is supposed to be faster than doing a jmp through a TSS. In fact if you look at the pcb structure it already has all the TSS structure there so making use of it would not change any kernel data structures. -- Brett Lymn